Why concentrated sulfuric acid for dehydration and dilute for hydration?

Question

To dehydrate ethanol one uses concentrated sulfuric acid: $$\\ce{C2H5OH ->[\ ext{conc.} H2SO4] C2H4 + H2O}$$ but to go in the reverse direction, dilute sulfuric acid is used: $$\\ce{C2H4 + H2O ->[\ ext{dil.} H2SO4] C2H5OH}$$ Why is one concentrated, and one dilute? For that matter, how does the sulfuric acid even help in the first place?

Answer 1

Le Chatlier's principle is in effect. Concentrated sulfuric acid is a mix of sulfuric acid and water as is dilute sulfuric acid having many more water molecules per sulfuric acid molecules in the mix. Dehydration is the removal of water from the ethanol and water mix. Finally ethanol is slightly acidic while sulfuric acid 0-H has greater acidity. The water partitions from the ethanol mix into the the concentrated sulfuric acid- hydrrogen bonding to two of the S=O lobes and diluting the acid while concentrating the ethanol. The dilute acid water in contact with dryer ethanol wets the ethanol: the water molecules hydrogen bond to the ethanol O-H pretty much as do water molecules create create cage structures between other water molecules. Not only do we have hydrogen bonding, we also have entropy effects. In both cases, the entropy of the system increases with the migration of water out of the dilute acid into the dryer ethanol and out of the wet ethanol into the concentrated acid. One can likely consider the forces as osmotic pressures where the water is considered as the solute; the sulfuric acid as the solvent and ethanol considered as the solvent- the partition being the layer in the extraction vessel between the acid and the ethanol. The extraction vessel being a seperatory funnel